<style>
  .countdown::part(separate-item) {
    color: #fff;
    background: red;
    padding: 0 8px;
    border-radius: 4px;
  }

  .countdown::part(separator) {
    margin: 0 4px;
    color: red;
  }
</style>

<b-countdown class="countdown" format="HHmmssSSS" time="7200000" separate>
  <span class="separator" slot="separator">:</span>
</b-countdown>